1. Field of the Invention
The present invention relates to an apparatus used for analyzing very small amounts of impurities, by total reflection fluorescent X-ray analysis, which are collected as a liquid drop on a sheet, in particular, on a semiconductor wafer.
2. Description of the Related Art
An apparatus using a fluorescent X-ray analysis method has been known as an apparatus for analyzing very small amounts of impurities deposited on a sheet. As the sheet, use is made of a quartz, a silicon, a plastic, and etc., sheet. Here, explanation will be given below, by way of example, about analyzing very small amounts of impurities deposited on a semiconductor wafer in particular.
As shown in FIG. 1, for example, upon the falling of an incident X-ray 32 on a silicon wafer 31 as a semiconductor wafer, a fluorescent X-ray 34 is produced from impurities 33 deposited on the silicon wafer 31. It is possible to identify elements from the wavelengths of the fluorescent X-ray 34 and to analyze amounts of elements from the intensities of the fluorescent X-ray. It is, therefore, possible to detect the number of atoms/cm.sup.2 in the impurity elements involved. The angle of incidence, .phi., of the incident X-ray 32 is greater than the so-called total reflection critical angle .phi. crit, that is, the angle of incidence at which a "total reflection" phenomenon occurs. The incident X-ray 32 penetrates into the interior of the silicon wafer 31 so that a fluorescent X-ray is produced from the silicon wafer 31 per se. In such a conventional fluorescent X-ray analysis method, there is a drawback in that signal noise is produced from that base material due to the X-ray involved and the so-called background emerges at an increasing level.
As a solution to this problem, a total reflection fluorescent X-ray analysis method as shown in FIG. 2 is known.
According to this method, an angle of incidence, .phi., of the incident X-ray 32 is made smaller than that used conventionally and, without penetrating into the silicon wafer 31, the incident X-ray is totally reflected on that surface. The incidence angle .phi. at this time is lower than the total reflection critical angle .phi. crit. This total reflection critical angle .phi. crit, though varying in the situations involved, is about below 1.degree..
The fluorescent X-ray reflected on the surface of the silicon wafer 31 in a 18 to 20 mm range is detected by a semiconductor detector. In this case, the normal impurity detection sensitivity E is about E=10 atoms/cm.sup.2. The semiconductor detector is fixed just over the surface of the silicon wafer 31. According to this method, it is possible to suppress the scattering of the incident X-ray and the rise of background by the fluorescent X-ray from the base material and hence to achieve high-sensitivity analysis.
When higher sensitivity analysis is required, the above-mentioned total reflection X-ray analysis method has conventionally been performed after the treatment of the silicon wafer surface with a hydrofluoric acid.
First, metal impurities deposited on the silicon wafer surface are decomposed with a hydrofluoric acid gas, a diluted hydrofluoric acid drop is scanned, as a liquid drop, on the surface to collect the metal impurities into the liquid drop, and the liquid drop is put on another silicon wafer surface to allow the water content of the liquid drop to be evaporated so that the metal impurities are dried to a solid. The solid thus obtained is analyzed by the total reflection fluorescent X-ray analysis method. According to this method, it is possible to provide a greater ratio between an analytical area and a dried solid area of the liquid drop and hence to improve the analytical sensitivity.
According to any above-mentioned methods, the fluorescent X-ray of silicon emerges as a main peak from the basic material as shown in FIG. 3. FIG. 3 shows how the intensity of the fluorescent X-ray reflected differs from element to element. That is, the number of X-ray detected during a time period of t seconds is represented by a cts unit.
An energy-distributed type semiconductor detector is used as high-sensitivity total reflection fluorescent X-ray detector but its energy resolution is of the order of one hundred and several tens of eV. When, therefore, a large peak of silicon emerges at 1.7 KeV, it is not possible to analyze the elements aluminum (Al) and phosphorus (P) because the intensity peaks of fluorescent X-rays of these elements Al and P overlap with that of silicon whose energy position is located 0.25 KeV away from that of Al and 0.27 KeV away from that of P. Further, due to the level of an S/N ratio involved, a rise in the intensity peak of the Si fluorescent X-ray leads to a rise in background of other impurity metals, so that it is difficult to achieve high-sensitivity analysis.
In contrast, attempts have been made to conduct analysis by coating an amorphous fluorine resin as a thin film on a silicon wafer and, putting a metal impurity-containing "liquid drop" on the coated silicon wafer, evaporating the water content of the liquid drop to a dried solid.
However, the thin film coated surface of the silicon wafer is liable to be degenerated through a thermal reaction with active chemicals, these being a risk that no requisite flatness will be obtained on total reflection fluorescent X-ray analysis. Further, the degeneration of the surface leads to poor heat conduction and a dried solid configuration of any liquid drop exhibits poor reproducibility on the surface. As appreciated from the above, difficulty has been encountered in the high-sensitivity analysis of the metal impurity on the thin film coated with the amorphous fluorine resin.